Predicting link quality of a link

ABSTRACT

A method and apparatus of predicting link quality of a link are disclosed. One exemplary method includes a receiver receiving multi-carrier modulated signals over a period of time. A signal to noise ratio (SNR) for each received sub-carrier is estimated. An ordered sequence of the signal to noise ratios (SNR)s are constructed based on interleaving of the multi-carrier modulated signals, wherein an order of the interleaving is used to set the ordered sequence of the signal to noise ratios (SNR)s. The receiver estimates link packet error rate (PER) based upon knowledge of encoding of the multi-carrier modulated signals used during transmission and the ordered sequence.

RELATED APPLICATIONS

This patent application is a continuation of patent application Ser. No.11/374,550 having the same title, filed Mar. 13, 2006 now U.S. Pat. No.7,440,412.

FIELD OF THE INVENTION

The invention relates generally to communication systems. Moreparticularly, the invention relates to a method and apparatus forpredicting transmission link quality.

BACKGROUND OF THE INVENTION

Ultra-wideband (UWB) modulation provides high data rate radiocommunications for transferring data using very wide modulationbandwidths. FIG. 1 shows a typical application of UWB communicationlinks used for indoor wireless communications. Several transceivers, forexample, transceivers 110, 120, 130, 140 are part of a network 100allowing high bandwidth communications between the transceivers 110,120, 130, 140. The transceivers 110, 120, 130, 140 can include, forexample, a high definition television (HDTV) monitor networked withother devices, such as, a digital video recorder (DVR), a digital videodisk (DVD) player and a computing device.

The Federal Communications Committee (FCC) has mandated that UWB radiotransmission can legally operate in the frequency range of 3.1 GHz to10.6 GHz. The transmit power requirement for UWB communications is thatthe maximum average transmit Effective Isotropic Radiated Power (EIRP)is −41.25 dBm/MHz in any transmit direction. The bandwidth of eachtransmission channel is 528 MHz.

Due to the low-power transmission associated with UWB communications, itis desirable to be able to predict link qualities between UWB devices.The links are subject to noise and interference, and as a result, canvary greatly in quality. A prior art method of determining link qualityincludes measuring signal to noise ratio (SNR) of transmission signals.However, in high-bandwidth transmission, SNR is not always a goodindicator of packet error rate (PER) of transmitted data. The noiseand/or interference can vary greatly over the large transmissionfrequency band. Two different transmission links having a similarmeasured average SNR can provide considerably different transmissiondata rate capacities. The optimal mode selection should provide the bestquality transmission link.

It is desirable to have a method and apparatus for predictingtransmission link quality for different transmission configurations. Itis desirable that the method and apparatus be efficient, and not requireadditional transmission bandwidth.

SUMMARY OF THE INVENTION

An embodiment of the invention includes a method of predicting linkquality of a link. The method includes receiving multi-carrier modulatedsignals over a period of time, estimating an SNR for each receivedsub-carrier, constructing a sequence of the SNRs, and estimating linkPER based upon knowledge of encoding used during transmission, and thesequence of SNRs.

Another embodiment of the invention includes a method of selecting atransmission mode of a transmission link based on a predicted linkquality of the link. The method includes receiving multi-carriermodulated signals over a period of time, estimating an SNR for eachreceived sub-carrier, constructing a sequence of the SNRs, estimatinglink PER for all possible transmission modes based upon knowledge ofencoding used during transmission and the sequence, and selecting atransmission mode based upon the link PER estimates.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 shows a prior art UWB mesh network.

FIG. 2 shows a configuration of a multi-carrier transmitter.

FIG. 3 shows a configuration of a multi-carrier receiver.

FIG. 4 is a flow chart showing an exemplary method of PER estimation ofa multi-carrier transmission signal.

FIG. 5 is a flow chart showing an exemplary method of estimating linkPER based upon knowledge of encoding used during transmission, andsequence of SNRs.

FIG. 6 is a flow chart showing an exemplary method of selecting atransmission mode of a transmission link based on a predicted linkquality of the link.

FIG. 7 shows a UWB mesh network that includes devices that include PERestimation or mode selection based on PER estimation.

DETAILED DESCRIPTION

The invention includes an apparatus and method for estimating packeterror rate (PER) of wireless links. The estimate uses knowledge oftransmitter coding, and estimates of the noise spectrum and transmissionchannel response based on received data. The estimates are determinedwithout requiring additional transmission bandwidth or overhead. PERestimates can be determined for multiple transmission data rates andmultiple time-frequency codes, based on characteristics of a singletransmitted data packet. The predictive estimates of PER for multiplelinks allows for transmission mode selection between a transmitter and areceiver based upon the estimates of PER.

Estimates of the PER can be made on multi-carrier signals (such as,orthogonal frequency division multiplexed (OFDM) signals). An exemplaryembodiment includes the PER estimate being made based upon transmissionchannel state information (channel coefficients), coding (rate andtype), noise power of the sub-carriers and the type of modulation.

FIG. 2 shows a configuration of a multi-carrier transmitter that canutilize the methods of estimating link transmission PER. The transmitterincludes a data encoder 210 and an interleaver 220. After the datastream has been encoded and interleaved, it is fed into a multi-carriermodulator 230. An exemplary multi-carrier modulator 230 is aninverse-fast-fourier-transform (IFFT), which can be used for OFDMsignals. More specifically, the IFFT can be an inverse discrete Fouriertransform. An exemplary IFFT converts block data samples from thefrequency domain to the time domain.

An embodiment of the encoder 210 includes forward error correction. Theforward error correction can include at least one of convolution coding,turbo coding, LDPC coding or block coding.

The interleaver 220 is typically designed to reduce the likelihood ofthe transmission channel introducing a burst of errors that cannot bespread into sparse errors. Single bit errors are much easier for adecoder to correct than burst errors. For dispersive channels, astypically seen in the environments in which UWB devices operate, thelikelihood of burst errors even with a well-designed interleaver, can besignificant. Therefore, if the receiver possesses knowledge of theinterleaver used by the transmitter, a better link PER estimate can becalculated.

Other exemplary transmitter functional blocks include a postfix inserter235 (for some types of OFDM signals), an interpolator/filter 240, adigital to analog converter (DAC) 250, a gain/filter block 255, afrequency up-converter 260, a band-pass filter 265, and one or moreantennas 270, 272. The two antennas 270, 272 are exemplary. That is, anydesired number of transmit antennas can be used.

As will be described, link PER estimations can be used to predicttransmission link quality for various transmission configurations. Areceiver (as will be described) can determine the PER estimates, or thereceiver in conjunction with the transmitter can determine the PERestimates. Based upon the PER estimates, desirable transmitter and/orreceiver configurations (also referred to as modes) can be selected. Theselections can be made on the encoding, the interleaving and the antennaconfiguration.

FIG. 3 shows a configuration of a multi-carrier receiver that canutilize the methods of estimating link transmission PER. The exemplaryreceiver includes receive antennas 310, 312 (a single antenna could beused, but multiple antennas can provide performance benefits). The twoantennas 310, 312 are exemplary. That is, any number of desired antennascan be used. The receiver can include a filter and amplifier (LNA) 315,a frequency down-converter 320, a gain and LPF block 325, an analog todigital converter (ADC) 330, a digital filter and decimator 335, an OFDMoverlap adder 340, a multi-carrier demodulator (for example, a fastfourier transform (FFT)) 345, a sub-carrier equalizer 350, ade-interleaver 355 and a decoder 360.

An exemplary controller 370 determines or calculates predicted link PERusing, for example, the methods described. The controller 370 of FIG. 3receives information regarding the interleaving (at transmitter), thecoding (encoding at transmitter). The controller 370 additionallyobtains estimates of the SNR of each sub-carrier (for example, duringthe preamble of OFDM signals) from an output of a channel and noiseestimator 365. The PER estimate can be used for mode selection as well.

The channel and noise estimator 365 provides the SNR per sub-carrierestimate. The estimator 365 receives data at the output of the FFT 345during preamble portions of the received packets. The SNR persub-carrier estimates can be updated through the payload of the receivedpackets, providing a more accurate estimate.

FIG. 4 is a flow chart of an exemplary method of estimating PER oftransmission signals based upon a single data packet. A first step 410of the method includes receiving multi-carrier modulated signals over aperiod of time. A second step 420 includes estimating an SNR for eachreceived sub-carrier. A third step 430 includes constructing an orderedsequence of the SNRs based on interleaving of the multi-carriermodulated signals. A fourth step 440 includes estimating link PER basedupon knowledge of encoding used during transmission and the sequence.

An advantage of this method is that the PER of a link can be estimatedbased on transmission of a single packet. By estimating the PER, andtherefore, the capacity of a link using only one packet, the linkthroughput can be optimized more quickly and consequently reduce thelatency. In addition, if the channel is being shared among multipledevices, determining the optimal throughput for each link in a timelymanner ensures that each device is transmitting on the channel a smallerfraction of the time, therefore, increasing the overall throughput ofthe network.

The PER of packets can be estimated over time. If the link quality ischanging, it can be advantageous to maintain a running average ofestimated link PER. The length of the running average can be based uponan environment subjected to the transmission channels. The more noisythe environment, the longer the average can be set. For example, if thetransmission is occurring in a home environment, people moving around aroom in which the transmission is occurring can cause the transmissionchannels to change rapidly. Here, a moving average estimate of PER maybe more useful. The length of the running average can be adaptive to howdynamic the channel appears to be.

Generally, the PER link estimate is made at a receiver of themulti-carrier modulated signals. That is, the link can be more easilycharacterized on the receiving end. The result can be fed back to thetransmitter. Alternatively, the receiver can feed back measured resultsof the received signals, and the results can be fed back to thetransmitter for characterization. The final PER estimate can be made ateither the receiving device, or at the transmitting device. Clearly, ifmade at the transmitting device, some information regarding the receivedmulti-carrier signals must be fed back to the transmitting device.

Receiving Multi-carrier Modulated Signals

An exemplary embodiment of transmission signals includes multi-carriermodulated signals. To alleviate the effects of ISI, an implementation ofUWB includes orthogonal frequency division multiplexing (OFDM) signaltransmission. OFDM is a special form of multi-carrier modulation inwhich multiple user symbols are transmitted in parallel using differentsub-carriers. The sub-carriers have overlapping frequency spectra, buttheir signal waveforms are specifically chosen to be orthogonal. OFDMsystems transmit symbols that have substantially longer time durationsthan the length of the impulse response of the transmission channel,thereby allowing avoidance of ISI. OFDM modulation techniques are veryefficient in indoor broad band wireless communication. It is to beunderstood that OFDM is one example of a multi-carrier transmissionsignal.

Estimating an SNR for Each Received Sub-carrier

An exemplary embodiment includes estimating SNR for each receivedcarrier based upon a transmission channel, noise variance andinterference. Since the SNR can vary from carrier to carrier, estimatingthe SNR on a carrier by carrier basis leads to the calculation of abetter PER estimate. For example, for a frequency selective transmissionchannel, the SNR can vary greatly between over just a few carriers of amulti-carrier signal.

A transmission channel of the multi-carrier signals can be estimatedbased upon knowledge of training signals transmitted through thechannel, and the received signals. Noise and interference of thetransmission channel can be estimated based upon the estimated channel,the training signals, and the received signals. The SNR of each carriercan then be estimated.

Constructing a Sequence of the SNRs

An exemplary embodiment includes the construction of the sequence SNRsbeing dependent upon the interleaving of the transmitter. That is, theorder of the interleaving can be used to set the order of the sequenceof the SNRs. If the sequence of SNRs is constructed by the receivingdevice, the receiving performs the construction based on knowledge ofthe interleaving within the transmitting device.

Estimating Link PER

An exemplary embodiment includes estimating link PER based uponknowledge of encoding used during transmission, and the sequence ofSNRs. FIG. 5 is a flow chart showing an exemplary method of estimatinglink PER based upon knowledge of encoding used during transmission, andsequence of SNRs. A first step 510 includes convolving the sequence withcodewords. The codewords are constructed based upon knowledge of theencoding at the transmitter. A second step 520 includes generating asequence of probability of error values by mapping each output of theconvolution. A third step 530 includes estimating link PER by summingthe probability of error values.

Convolving with Codewords

Generally, there are a predetermined number of codewords. A library canbe used to reference the predetermined number of codewords. The totalnumber of possible codewords for a transmission system is dependent uponthe length (number of bits) of the interleaver, the length (number ofbits) of the packets and the code. Generally, only a subset of the totalnumber of codewords is actually selected for use. Generally, the set(library) of codewords that are selected are the codewords that aredetermined to be the most dominant contributors in determining PERestimates. The PER contributions for each codeword, can be determined byconvolving the codewords with the sequence of SNRs. A subset of thecodewords is typically selected to reduce complexity.

An exemplary embodiment includes the codewords having entries of 1s and0s. Therefore, the convolution includes summing some (as determined bywhich entries are a 1) of the SNRs of the sequence of the SNRs. Otherembodiments include codewords having values different than 1 or 0. Anexemplary embodiment of convolving the sequence with codewords, includesconvolving the sequence with each codeword of the library of codewords.

Generating a Sequence of Probability of Error Values

The convolution results in a sequence in which the values of theelements within the sequence are dependent upon the values of thesequence of SNRs and the values within the codewords. Each of the valuesof the sequence can be mapped to provide a sequence of probabilities oferror. An exemplary embodiment includes the mapping being performedthrough the use of a look up table (LUT). That is, a probability oferror is generated by the LUT for each value of the sequence. The LUTmaps the value of each element of the sequence to a probability oferror.

The LUT can be generated, for example, by approximating the output of aQ function for all possible LUT input values. The Q-function is wellknown in communication systems, and includes a finite integral of aGaussian probability density function.

Each entry in the sequence provides an estimate of a probability oferror at that point in the sequence. Packets can be defined to include Mbits. The interleaver can be defined to include N bits. The number ofbits within the packets can be M/N or X times the number of bits withinthe interleaver. Since constructing a sequence of SNRs of length Ninvolves X repetitions of a unique sequence of SNRs of length N(assuming the SNR per sub-carrier does not change in over the durationof one packet), the convolution can be performed using only a length NSNR sequence and then multiplying the resulting probability of error bya factor of X. Generally, the convolution is circular to avoid “edge”effects.

The PER estimate can be scaled by the ratio of packet length M tointerleaver length N.

Summing Probability of Error Values

The PER of the link can be estimated by summing the probability of errorvalues of the sequence.

Mode Selection Based on Link PER Estimates

The methods of estimating link quality through PER estimates can be usedfor selecting transmission modes between a transmitter and a receiver.That is, generally the transmitter has several different transmissionmodes that include, for example, multiple transmission rates, multipletypes of coding, interleaving, multiple configurations of transmitterantennas and/or multiple configurations of receiver antennas.

An embodiment of the transmission includes mode selections that includeselecting a frequency hopping sequence for a multi-band transmissionsystem. The transmission link quality can vary depending on thetransmitted frequency hopping sequence due to the dependence onfrequency selectivity of the transmission channel, the interleaver andthe encoder. The PER estimator accurately estimates link quality foreach of the possible frequency hopping sequences. Therefore, thetransmitter can select the best frequency hopping sequence based on thePER estimates.

Multiple antennas can exist at both the transmitter and the receiverproviding multiple input, multiple output (MIMO) communication. Thecommunication can include diversity or spatial multiplexing.

Diversity communication can be used to minimize the effects of multipathand interference. Essentially, multiple versions of the same signal aretransmitted between a transmitter and receiver. An embodiment ofdiversity includes receive antenna select diversity. Specifically, oneof multiple receive antennas is selected for reception.

An embodiment of the receiver includes phase combining. Diversitycombining is generally the preferred method of diversity reception whenthe received signals are correlated. If diversity combining isdetermined to be desirable, the optimal phase relationship between thereceived diversity signals must be determined. This determination can bedependent upon whether the received signals include proportionallylarger noise distortion, or signal interference distortion.

Spatial multiplexing is a transmission technology that exploits multipleantennas at both transmitter and receiver to increase the bit rate in awireless radio link with no additional power or bandwidth consumption.Under certain conditions, spatial multiplexing offers a linear increasein spectrum efficiency with the number of antennas.

An embodiment of the receiver includes Maximal Ratio Combining (MRC).Generally, MRC includes independently weighting signals received by eachantenna of a multiple antenna receiver, and combining the weightedsignals. Typically, the weighting is selected to maximize a signal tonoise (SNR) of the combined signals. For multi-carrier signals, theweighting can be performed independently on each sub-carrier signal.

Typically, the transmission mode that is selected, is the transmissionmode that provides the link have the best predicted PER.

FIG. 6 is a flow chart that shows an exemplary method of selecting atransmission mode of a transmission link based on a predicted linkquality of the link. A first step 610 of the method includes receivingmulti-carrier modulated signals over a period of time. A second step 620includes estimating an SNR for each received sub-carrier. A third step630 includes constructing an ordered sequence of the SNRs based oninterleaving of the multi-carrier modulated signals. A fourth step 640includes estimating link PER for all possible transmission modes basedupon knowledge of encoding used during transmission and the sequence. Afifth step 650 includes selecting a transmission mode based upon thelink PER estimates.

One embodiment can include the receiver selecting the mode, and feedingthe mode selection back to the transmitter. Another embodiment includesthe receiver receiving the multi-carrier modulated signals, andperforming only a subset of the processing. The semi-processed signalsare fed back to the transmitter, and the transmitter makes the finaltransmission mode selection.

If the transmission channel between transmitter and the receiver areapproximated to be the same in both transmission directions, then thetransmitter can receive signals from the receiver and make the modeselection. That is, reciprocity of the transmission channel between thetransmitter and the receiver is assumed. The transmitter then makes amode selection based on one packet received from the receiving(transceiver) device. Situations can occur which do not allowreciprocity to be assume. For example, reciprocity can not be assumedif, for example, an interferer is located proximate to the receiver. Inthis situation, the transmitter can not properly estimate the SNRs persub-carrier as received at the receiver. In this situation, the receivershould either estimate the PER and feed it back to the transmitter, orat least feed back the SNRs per sub-carrier.

FIG. 7 shows networked components that can benefit from the use oftransmission pre-processing that include multi-carrier time spreading.The network can include, for example, a high definition television(HDTV) monitor 710 networked with other devices, such as, a digitalvideo recorder (DVR) 720, a digital video disk (DVD) player 740 and acomputing device 730. Each of the components 710, 720, 730, 740 includesPER estimations and/or mode selection based upon PER estimations as havebeen described.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited only by the appended claims.

1. A method of predicting link quality of a link, comprising: a receiverreceiving multi-carrier modulated signals over a period of time;estimating a signal to noise ratio (SNR) for each received sub-carrier;constructing an ordered sequence of the signal to noise ratios (SNR)sbased on interleaving of the multi-carrier modulated signals, wherein anorder of the interleaving is used to set the ordered sequence of thesignal to noise ratios (SNR)s; the receiver estimating link packet errorrate (PER) based upon knowledge of encoding of the multi-carriermodulated signals used during transmission and the ordered sequence. 2.The method of claim 1, further comprising de-interleaving the orderedsequence of signal to noise ratios (SNR)s based upon knowledge oftransmission interleaving.
 3. The method of claim 1, wherein estimatingsignal to noise ratio (SNR) for each received sub-carrier is based upona transmission channel, noise variance and interference.
 4. The methodof claim 1, wherein the multi-carrier modulated signals are orthogonalfrequency division multiplexed (OFDM) signals.
 5. The method of claim 1,wherein estimating link packet error rate (PER) is performed per packet.6. The method of claim 5, further comprising: estimating link packeterror rate (PER) of packets over time, and maintaining a running averageof estimated link packet error rate (PER).
 7. The method of claim 6,wherein length of the running average is based upon how rapidlytransmission channels are varying.
 8. The method of claim 1, wherein thepacket error rate (PER) link estimate is made at a receiver of themulti-carrier modulated signals.
 9. The method of claim 1, wherein thepacket error rate (PER) link estimate is made at a transmitter of themulti-carrier modulated signals based on feedback from a receiver of themulti-carrier modulated signals.
 10. The method of claim 8, wherein atleast one of the receiver and a transmitter comprises multiple receiveantennas.
 11. The method of claim 10, wherein the receiver comprisesmaximal ratio combining.
 12. The method of claim 10, wherein thereceiver comprises phase combining.
 13. The method of claim 10, whereinthe receiver comprises receive antenna select diversity.
 14. The methodof claim 1, wherein the encoding comprises forward error correction(FEC) coding comprising at least one of convolutional, turbo and lowdensity parity check (LDPC) coding.
 15. A method of selecting atransmission mode of a transmission link based on a predicted linkquality of the link, comprising: a receiver receiving multi-carriermodulated signals over a period of time; estimating a signal to noiseratio (SNR) for each received sub-carrier; constructing an orderedsequence of the signal to noise ratios (SNR)s based on interleaving ofthe multi-carrier modulated signals, wherein an order of theinterleaving is used to set the ordered sequence of the signal to noiseratios (SNR)s; the receiver estimating link packet error rate (PER) forall possible transmission modes based upon knowledge of encoding of themulti-carrier modulated signals used during transmission and the orderedsequence; selecting a transmission mode based upon the link packet errorrate (PER) estimates.
 16. The method of claim 15, wherein thetransmission mode that is selected is the mode that corresponds with thebest packet error rate (PER) estimate.
 17. The method of claim 15,wherein the transmission modes comprise at least one of multiple datatransmission rates, multiple codes, multiple transmit antennas andmultiple receive antennas.
 18. A receiver comprising: means forreceiving multi-carrier modulated signals over a period of time; meansfor estimating an signal to noise ratio (SNR) for each receivedsub-carrier; means for constructing an ordered sequence of the signal tonoise ratios (SNR)s based on interleaving of the multi-carrier modulatedsignals, wherein an order of the interleaving is used to set the orderedsequence of the signal to noise ratios (SNR)s; means for estimating linkpacket error rate (PER) based upon knowledge of encoding used duringtransmission, and the sequence.
 19. The receiver of claim 18, whereinthe receiver feeds the estimated link packet error rate (PER) back to atransmitter.
 20. The receiver of claim 18, further comprisingde-interleaving the sequence of signal to noise ratios (SNR)s based uponknowledge of transmission interleaving.